This invention relates to a method and apparatus for anti-aliasing an image boundary during video special effects, particularly in order to improve the outline of an image created by digital video coordinate transformation effects.
In a video special effects apparatus a first scene, for example as shown in FIG. 1(a), may be used to occlude part of a second scene (FIG. 1(b)), so that a composite scene (FIG. 1(c)) is generated in which the first scene appears as a foreground against a background of the second scene. In a known digital video effects apparatus, this is accomplished by using a foreground video signal representing the first scene to provide a first rectangular array of digital values and a background video signal representing the second scene to provide a second rectangular array of digital values. A crop signal is used to provide a third two-dimensional array of values. The three arrays of digital values exist in mutually coextensive domains, corresponding to the active area of a video frame, and the sampling intervals for the three arrays are the same. Thus, for each element of one array there are corresponding elements in the other arrays. The crop signal is used to control combination of the first and second arrays. In particular, each element of the crop array is multiplied by the corresponding element of the foreground array and its complement is multiplied by the corresponding element of the background array, and the two resulting elements are added together. Over most of the frame, the crop signal has one of two values, corresponding to a multiplication factor of one and a multiplication factor of zero respectively. Thus, the crop signal represents a geometrical figure whose periphery defines the boundary in the composite scene between the foreground and the background.
Both the foregound and background video signals are bandwidth-limited in order to avoid aliasing when the array which results from combining the foreground and background arrays is used to reconstruct a video signal representing the composite scene.
In a conventional digital video effects apparatus, the crop signal is processed in identical fashion to the foreground video signal. This ensures that if, for example, the first scene is rotated, translated or compressed, the figure defined by the crop signal is rotated, translated or compressed in identical fashion.
When the first scene is compressed, so that an object of the foreground occupies a reduced area in the composite scene, "jaggies", which are caused by aliasing, are sometimes observed in the composite scene along the boundary between the foreground and the background, even though the foreground signal has been filtered to take account of the increase in bandwidth that results from spatial compression.